1. Field of the Invention
Embodiments of the present invention generally relate to thin film transistors (TFT) and methods for fabrication thereof.
2. Description of the Related Art
Current interest in TFT arrays is particularly high because these devices may be used in liquid crystal active matrix displays (LCDs) of the kind often employed for computer and television flat panels. The LCDs may also contain light emitting diodes (LEDs), such as organic light emitting diodes (OLEDs) for back lighting. The LEDs and OLEDs require TFTs for addressing the activity of the displays. The current driven through the TFTs (i.e., the on-current) is limited by the channel material (often referred to as the active material, semiconductor material or semiconductor active material) as well as the channel width and length. Additionally, the turn-on voltage is determined by the accumulation of the carrier in the channel area of the semiconductor layer which could change as the shift of the fixed charge in the semiconductor material or the charge trapping in interfaces and the threshold voltage shifts after bias temperature stress or current temperature stress.
The current practice in making the channel or semiconductor layer in a TFT is to deposit the semiconductor layer at a low temperature and then anneal the semiconductor layer at a higher temperature to increase the stability and mobility of the semiconductor layer. The annealing temperature is limited by the temperature that may be tolerated by the substrate. The performance of the TFT is limited by the deposition and annealing temperatures. The semiconductor layer may be deposited through a low temperature deposition process such as physical vapor deposition (PVD), chemical vapor deposition (CVD), sputtering, spin on processes, etc., but the semiconductor layer needs to either be annealed to stabilize the film structure and achieve better performance for the electronic device or have a passivation layer deposited thereover. For example, changes in film properties, such as mobility and carrier concentration, have been observed after several weeks of exposure to air for some zinc oxynitride films deposited at 50 degrees Celsius (note that the films have either not been annealed at a high temperature or do not have a passivation layer formed thereover).
Therefore, there is a need in the art for a method of fabricating a semiconductor layer for a TFT in which the device performance is not limited by the annealing and deposition temperatures.